1. Field of the Invention
The present invention relates to laminated flat cables and methods for producing the same, and more particularly, the present invention relates to a laminated flat cable for use in high-frequency signal transmission and a method for producing the same.
2. Description of the Related Art
As a conventional laminated flat cable, a high-frequency signal line described in, for example, Japanese Utility Model No. 3173143 is known. FIG. 14 is an exploded view of the high-frequency signal line 500 described in Japanese Utility Model No. 3173143.
The signal line 500 shown in FIG. 14 includes a dielectric element assembly 512, a ground conductor 530, a signal line 532, and an auxiliary ground conductor 534. The dielectric element assembly 512 is formed by laminating dielectric sheets 522a to 522c in this order.
The signal line 532 is provided on the dielectric sheet 522b. The ground conductor 530 is provided on the dielectric sheet 522a so as to be opposite to the signal line 532 with the dielectric sheet 522a positioned therebetween. Moreover, the ground conductor 530 has a plurality of openings 540 overlapping with the signal line 532.
The ground conductor 534 is provided on the dielectric sheet 522c so as to be opposite to the signal line 532 with the dielectric sheet 522b positioned therebetween.
In the signal line 500 thus configured, since the ground conductor 530 has the openings 540 provided therein, less capacitance is generated between the ground conductor 530 and the signal line 532. Accordingly, even if the distance between the ground conductor 530 and the signal line 532 in the direction of lamination is reduced, the capacitance generated therebetween is inhibited from becoming excessively high and thus causing the characteristic impedance of the signal line 532 from deviating from a desired impedance value. Thus, the signal line 500 allows the dielectric element assembly 512 to be reduced in thickness.
However, in the signal line 500 described in Japanese Utility Model No. 3173143, there might be variability in characteristic impedance of the signal line 532, as will be described below. More specifically, in the signal line 500, the ground conductor 530 is a conductor having the openings 540 provided therein, and the ground conductor 534 is a conductor in the form of a solid without openings. At the time of designing the signal line 500, the distance between the signal line 532 and the ground conductor 534 is decided such that the characteristic impedance of the signal line 532 is higher than a predetermined impedance value (e.g., 50Ω) in a state where the ground conductor 534 is present but the ground conductor 530 is not present. Thereafter, the shape of the opening 540 in the ground conductor 530 and the distance between the signal line 532 and the ground conductor 530 are decided such that the characteristic impedance of the signal line 532 has a predetermined impedance value (e.g., 50Ω) in a state where the ground conductor 530 is added.
Here, in the state where the ground conductor 534 is present but the ground conductor 530 is not present, the characteristic impedance of the signal line 532 is determined by the distance between the signal line 532 and the ground conductor 534. Therefore, the signal line 500 is required to be produced such that the distance between the signal line 532 and the ground conductor 534 satisfies the range of design values.
However, the signal line 532 is provided on the dielectric sheet 522b, whereas the ground conductor 534 is provided on the dielectric sheet 522c. Accordingly, the distance between the signal line 532 and the ground conductor 534 might be out of the range of design values if the dielectric sheets 522b and 522c are not laminated in such a manner as to contact evenly with each other. As a result, there might be variability in characteristic impedance of the signal line 532.